1. Field of the Invention
This invention generally relates to wireless communications antennas and, more particularly, to a selectable antenna array formed from a microelectromechanical switch.
2. Description of the Related Art
The size of portable wireless communications devices, such as telephones, continues to shrink, even as more functionality is added. As a result, the designers must increase the performance of components or device subsystems while reducing their size, or placing these components in less desirable locations. One such critical component is the wireless communications antenna. This antenna may be connected to a telephone transceiver, for example, or a global positioning system (GPS) receiver.
Wireless telephones can operate in a number of different frequency bands. In the US, the cellular band (AMPS), at around 850 megahertz (MHz), and the PCS (Personal Communication System) band, at around 1900 MHz, are used. Other frequency bands include the PCN (Personal Communication Network) at approximately 1800 MHz,
the GSM system (Groupe Speciale Mobile) at approximately 900 MHz, and the JDC (Japanese Digital Cellular) at approximately 800 and 1500 MHz. Other bands of interest are GPS signals at approximately 1575 MHz and Bluetooth at approximately 2400 MHz.
Conventionally, good communication results have been achieved using a whip antenna. Using a wireless telephone as an example, it is typical to use a combination of a helical and a whip antenna. In the standby mode with the whip antenna withdrawn, the wireless device uses the stubby, lower gain helical coil to maintain control channel communications. When a traffic channel is initiated (the phone rings), the user has the option of extending the higher gain whip antenna. Some devices combine the helical and whip antennas. Other devices disconnect the helical antenna when the whip antenna is extended. However, the whip antenna increases the overall form factor of the wireless telephone.
It is known to use a portion of a circuitboard, such as a dc power bus, as an electromagnetic radiator. This solution eliminates the problem of an antenna extending from the chassis body. Printed circuitboard, or microstrip antennas can be formed exclusively for the purpose of electromagnetic communications. These antennas can provide relatively high performance in a small form factor. However, a wireless device that is expected to operate at a plurality of different frequencies may have difficulty housing a corresponding plurality of microstrip antennas. Even if all the microstrip antennas could be housed, the close proximity of the several microstrip antennas may degrade the performance of each antenna.
In some circumstances it is advantageous to be able to shape an antenna pattern. Then, the antenna pattern has additional gain in a desired direction, to improve the link margin with a communicating device. It is known to network a plurality of antenna elements and regulate the phase relationship between elements. The phase relationship between elements generates the antenna beam pattern. Likewise, an active element can be arrayed in a field, or lattice of parasitic elements. A lattice is a substantially symmetrical arrangement having two or more members. These parasitic elements, being either half-wavelength open radiators or quarter-wavelength ground-shunted radiators, can also be used to shape an antenna beam pattern. Unlike the phase-array antenna, whose pattern can easily be varied by electronic means, the parasitic elements must be manipulated by mechanical means if the beam is to shaped in a different form. Mechanical manipulation generally requires additional parts that take up room and degrade reliability. As a result, parasitic element lattices have not been practical for use in portable wireless communication devices.
FIG. 20 is a schematic diagram of a microelectromechanical switch (MEMS) (prior art). A MEMS is a semiconductor integrated circuit (IC) with an overlying mechanical layer that operates as a selectable connectable switch. That is, the underlying solid-state layer creates a field that can cause an overlying conductive material to move, permitting the conductive material to act as miniature single-pull single-throw switch. MEMS concepts were developed in labs in the 1980's and are just now beginning to be fabricated as practical products. As a result, the particular specifications and features of a MEMS are still under development. MEMS technology offers the possibility of extremely low loss switches miniature switches.
It would be advantageous if a single wireless communications telephone antenna could be made to operate at a plurality of frequencies using MEMS devices.
It would also be advantageous if the antenna beam pattern of the above-mentioned multi-frequency MEMS antenna could be controlled.
It would be advantageous if the MEMS devices could be used to vary the electrical length of parasitic elements in a parasitic element antenna array.